The subject matter disclosed herein relates generally to a switching power supply and, more specifically, to a system and method for controlling paralleled switching devices in a power supply.
As is known to those skilled in the art, switching power supplies utilize a power semiconductor device, such as a thyristor, silicon-controlled rectifier (SCR), or one of several types of transistors, such as insulated-gate bipolar transistors (IGBTs) or metal-oxide-semiconductor field-effect transistors (MOSFETs) to regulate power flow between an input and an output of the power supply. The power semiconductor devices may be arranged in numerous configurations such as boost, buck, half bridge, or full bridge arrangements to convert and/or regulate the voltage provided at the input to a desired voltage at the output of the power supply.
Typically a processor generates gating signals at a periodic frequency, also referred to as the switching frequency, to control operation of the power semiconductor, or switching, devices. The gating signals cause each device to conduct or block voltage through the device at certain times during the switching period. The switching devices are often paired with a diode, reverse-connected across the switching device, and with reactive elements, such as capacitors or inductors to maintain a desired voltage level and/or a desired current flow through the power converter for short durations when the switching device is turned off. Appropriate modulation techniques can generate gating signals to increase or decrease the voltage level between the input and the output as well as convert the voltage between an alternating current (AC) voltage and a direct current (DC) voltage.
As is also known to those skilled in the art, the switching devices in a switching power supply generate power losses within the power supply. Two common losses are switching losses and conduction losses. Switching losses result from a sudden transient condition and resultant spike in voltage and/or current across the switching device when the device transitions between an “on” and an “off” state. Conduction losses result from power dropped across the device due to the amplitude of current conducted and the inherent resistive properties of the switching device. As the power ratings of the switching devices increase, the corresponding power loss in the device similarly increases. Further, as the switching frequency increases, the number of transitions increases which, in turn, increases the switching losses across the device. Due to the increased power lost in switching devices as the ratings and switching frequencies increase, the switching power supply requires increased cooling and/or heat dissipation capacity of the switching power supply. Increasing the cooling or heat dissipation capacity results in additional expense and/or size for the power supply.
Historically, it has been known to connect multiple switching devices in parallel to provide switching power supplies having an increased overall power rating, yet allowing each of the switching devices to dissipate a portion of the power losses. Ideally, switching each of the paralleled devices in unison results in the switching and conduction losses being distributed evenly among the devices. However, due to manufacturing tolerances and transmission delays within control circuits, even if the same gating signal is provided to each device, there is typically some variation in the time at which each device turns on and/or off. As a result, one of the devices bears the majority of the switching losses and is likely to fail prior to the other devices. Many techniques have been developed to synchronize switching of the paralleled devices. However, the synchronization routines have varying levels of success and add undesirable complexity to controlling the switching devices.
Thus, it would be desirable to provide an improved system and method for controlling multiple, paralleled switching devices in a switching power supply.